Method for assembling an optical array comprising coaxial shells

ABSTRACT

The invention provides a method of assembling an optical assembly having first and second longitudinal ends and comprising N coaxial shells forming individual mirrors, each of which extends between said first and second ends and presents a first diameter at said first end and a second diameter that is greater than the first at said opposite, second end, the method comprising: 
     1) putting the first end of the first shell situated outermost in the optical assembly into place on a support; 
     2) putting the first end of the second shell which is immediately adjacent thereto in the optical assembly into place on the support inside the first shell; . . . ; and 
     N) putting the first end of the Nth shell which is situated innermost in the optical assembly into place on the support.

The present invention relates to a method of assembling an opticalassembly having first and second longitudinal ends and comprising Ncoaxial shells forming individual mirrors, each of which extends betweensaid first and second ends and presents a first diameter at said firstend and a second diameter that is greater than the first at saidopposite, second end, where the shells can be complete cylinders orcylindrical segments.

BACKGROUND OF THE INVENTION

Such an optical assembly is known in particular as the WOLTER I typetelescope mirror in which each individual mirror is a mirror for X-raysat grazing incidence, and is in the form of a surface of revolutionhaving a region in the form of a parabola of revolution (adjacent to thelarger-diameter second end) and a region in the form of a hyperbola ofrevolution (adjacent to the smaller-diameter first end).

Such an assembly and its method of integration is described in anarticle by D. de Chambure et al. entitled “Producing the X-ray mirrorsfor ESA's XMM spacecraft”, published in ESA Bulletin No. 89 of February1997, pages 68 to 79.

During integration, each shell, starting with the centermost shell, ismeasured and then positioned by its second end and fixed on a support,integration being performed from the center outwards.

The optical performance of each individual shell must be optimized priorto integration, which requires manufacture to very high standards ofquality.

After integration, it is possible to monitor the optical performance ofeach shell forming the mirror, but it is not possible to make individualcorrections to each shell. Unfortunately, the integration operationgives rise to deformation of the individual mirrors, if only because ofgravity.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of interactionwhich makes it possible to perform measurements and possibly to makecorrections each time a new shell is integrated.

The invention thus provides a method of assembling an optical assemblyhaving first and second longitudinal ends and comprising N coaxialshells forming individual mirrors, each of which extends between saidfirst and second ends and presents a first diameter at said first endand a second diameter that is greater than the first at said opposite,second end, the method comprising:

1) putting the first end of the first shell situated outermost in theoptical assembly into place on a support;

2) putting the first end of the second shell which is immediatelyadjacent thereto in the optical assembly into place on the supportinside the first shell; . . . ; and

N) putting the first end of the Nth shell which is situated innermost inthe optical assembly into place on the support.

Since the shells are integrated starting from the outermost shell andgoing inwards, with the shells being held on the support at least viatheir smaller-diameter ends, the inside surfaces of the shells, i.e.their active reflecting surfaces, remain accessible until the next shellis put into place, so it is thus possible to perform any corrective oradditional operation that might be found suitable on each shell.

In particular, at least one of said installation operations comprises:

a) positioning one of said shells on the support;

b) measuring the topography of the inside surface of said shell aspositioned on the support;

c) where appropriate, repositioning said shell on the support as afunction of the result of said topography measurement; and

c′) fixing the position of said shell on the support.

In a preferred variant, at least one of said installation operationsincludes, after said fixing of its position on the support:

d) measuring the topography of the inside surface of said shell fixed onthe support; and

e) where appropriate, ion machining the inside surface of said shell.

After e), it is particularly advantageous for the method to comprise:

f) applying a reflective coating on the inside face of said shell, andoptionally, after f):

g) optically verifying said shell.

In said method, said topographical measurement is preferably implementedby differential measurement by scanning both, the inside surface of saidshell and a reference cylinder placed on the support in a referenceposition, said differential measurement being performed without makingcontact by means of sensors which are carried by a measurement platewhose displacements are identified relative to said reference cylinder.

At least one shell can present at least one extension to at least one ofits longitudinal ends.

In the method at least one shell can be constituted by a plurality ofelements extending between the first and second ends, each elementoccupying a portion of the outline of said shell, and said elements canpresent at least one extension disposed at least one of the longitudinalends thereof and at least one of the side edges thereof.

Such extensions constitute mechanical fixing elements. At least one ofsaid extensions disposed at a longitudinal end can constitute a bafflefor attenuating parasitic light.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will appear betteron reading the following description given by way of non-limitingexample and with reference to the accompanying drawings, in which:

FIG. 1 shows a module for the XMM telescope;

FIGS. 2a to 2 c show the integration method of the invention;

FIG. 3 shows a measuring device adapted to the method of the invention;

FIG. 4 shows one way of making a portion of an individual mirror; and

FIG. 5 shows one way of making an individual mirror.

MORE DETAILED DESCRIPTION

The present trend in space astronomy is to develop optical systemshaving a large collecting surface area and resolution of less than 1second of arc. In general, this implies manufacturing a large number ofhigh quality mirrors which operate in a thermally stabilizedenvironment, having gradients of less than 0.2° C. and at temperaturesthat may be as low as −80° C. One of the problems posed by such mirrorsis their manufacturing cost.

The present invention proposes a method of integrating mirrors that isparticularly although not exclusively suitable for an optical system 1implementing WOLTER I type mirrors, operating in the energy band lyingbetween 0.003 keV an 100 keV (i.e. wavelengths lying in the range 400 nmto 0.01 nm). Individual circularly symmetrical mirrors or shells (M₁, .. . , M_(n)) each having an inlet region 2 of parabolic sectionpresenting an inlet end 4, and an outlet region 3 of hyperbolic sectionpresenting an outlet end 5, are assembled together to form a module 10of concentric mirrors sharing a common focus, with each mirror beingsuitable for receiving X-rays at grazing incidence in the direction ofarrow F. Each individual mirror (M₁, . . . , M_(n)) is a thin mirrorwith such a mirror being defined as having a ratio of thickness overmean radius of curvature that is less than {fraction (1/50)}.

Downstream from the module 10, there is placed a dispersive grating 11and two charge-coupled device (CCD) sensors 12 and 14 for picking upX-rays, respectively non-dispersed X-rays and dispersed X-rays.

A technique for manufacturing and integrating such mirrors is describedin the above-mentioned article by D. de Chambure.

The problems raised by integrating such mirrors are the following:

it is difficult to perform on-site measurements of the installed opticalsurfaces;

the optical system is deformed as the mirrors are integrated, eventhough the mirrors need to be manufactured to their final specificationswhich are very tight, thereby giving rise to high costs;

differential deformation due to different thermal expansion coefficientstake place between the mirrors and the support during various stages:manufacture; integration; testing; and use; and

it is difficult to bring all of the individual mirrors into properalignment so as to cause their focuses to coincide.

The present invention proposes a method that enables integration to beimproved and allows for final corrections to be made to the individualmirrors when building up a module.

In the method, the mirrors are integrated on a support 20 in Nsuccessive stages, starting by the largest-dimension mirror M₁ (see FIG.2a), i.e. the outermost mirror of the module 10, with the mirror havingits first end or downstream end 5, i.e. its smaller-diameter end, placedon the support, and then proceeding step by step (M₁, M₂, M₃, . . . ) tothe Nth mirror which is likewise stood on its downstream end 5 (seeFIGS. 2b and 2 c).

As a result, the internal reflecting surface 6 of each shell 1 that hasjust been integrated on the support 20 is still accessible for measuringthe shell which has just been integrated, by using a device (34, 35)described below (with reference to FIG. 3) and for making anycorrections.

It is possible to deform the support 20 so as to compensate for theadditional load due to the weight of the individual mirrors as they areintegrated in succession, or indeed to turn the support 20 in such amanner as to take account of any difference that might exist between theoptical axis of a mirror to be integrated and the vertical axis.

The individual mirrors can be corrected by subjecting each mirror to ionpolishing after it has been integrated. This makes it possible tocompensate for manufacturing defects in the mirrors and/or for defectsdue to the integration process (shrinkage of adhesive, mechanicalloading, etc. . . . ). Ion polishing presents the advantage of notdegrading the microroughness of the polished surfaces, providing therate of removal and the quantity of matter removed are kept withinreasonable limits. This method of correction is also a method thatavoids contact and does not have side effects.

One solution for reducing the deformation generated within theindividual mirrors is to fix them via an interface region that is notactive optically, thereby making it possible to attenuate stresses.

For example, FIG. 4 shows an individual mirror of WOLTER I type which ismade up of elements 40 constituting cylindrical segments occupying afraction of a full turn, with each presenting a region 42 of parabolicsection and a region 43 of hyperbolic section. The edge 44 of the region42 is extended by a tab 46 for fixing to a part that covers the mirrorassembly, while the edge 45 of the region 43 is extended by a tab 47 forfixing to the support 20. Laterally, and on at least one side, theregions 42 and 43 are extended by respective fixing tabs 48 and 49.These mechanical fixing tabs constitute ends which are integral with theindividual mirrors.

FIG. 5 shows an individual mirror of the WOLTER I type that forms acomplete cylinder, and that presents upstream and downstream fixing tabs56 and 57, which are connected via upstream and downstream edges 54 and55 to a region 52 of parabolic section and to a region 53 of hyperbolicsection.

The tabs 46 to 49, 56, and 57 can enable temperature to be controlledvery close to the optical system.

The tabs 46, 47, 56, and 57 also make it possible to limit the quantityof parasitic light that penetrates into the module.

The presence of parasitic, interfering light is inherent to telescopeshaving a grazing angle of incidence. In order to attenuate suchparasitic light, it is known to place baffles or screens that are inco-alignment with the mirrors, and these baffles are difficult tomanufacture and difficult to bring into alignment, so they are expensiveand time consuming. One such optical baffle can be made integrally withan individual mirror, e.g. by electroforming. It is then possible afterintegration to treat the optical baffle situated at the upstream end 4,while the mirror is standing on its downstream end 5. This machiningtreatment for imparting controlled roughness to the inside surface ofthe baffle can be performed by ion machining, during the operation ofapplying ion machining to the reflecting surface of the mirror.

After an individual mirror has been integrated, it is possible to coatit in a conventional coating to give it characteristics of highreflectivity over a wide passband. Such coating is implemented byapplying one or more layers, e.g. layers of metal.

The mirror support 20 (cf. FIG. 3) has a device 39 for compensating thedeformation induced by the weight of the individual mirrors as they areintegrated in succession. The support 20 carries a reference cylinder 33which faces the optical surface 37′ of the mirror 37 which has just beenintegrated and whose axis 33′ is preferably parallel to the commonoptical axis X of the individual mirrors (M₁, . . . , M_(n)).

The mirrors are held at points that are distributed around their edges(possibly equally distributed) and they are lowered parallel to the axisX by using the cylinder 33 as a reference for the horizontal axes Y andZ so as to ensure that the mirror being integrated is deposited afterfollowing a required trajectory which enables it to be put down withouttouching any of the previously-integrated mirrors. It is possible to usethe mirror-handling tool that is described in the article by D. deChambure et al. entitled “The status of the X-ray mirror production forthe XMM spacecraft”, published in SPIE Proceedings No. 2808, pages362-375 (1966).

Once the mirror is in place, the topography of its active surface 37′ ismeasured by scanning using contactless gauges, and the referencecylinder 33. Topography can also be measured by an optical test.

After a scan, the optimal position for the mirror 37 is calculated andthe handling tools reposition it, should that be necessary.

The mirror 37 is then fixed in position by adhesive or by mechanicalfixing, e.g. by screws. The handling tool is then decoupled from themirror 37. At this moment, the weight of the mirror 37 is transferred tothe support 20, thereby deforming it. This deformation is measured andthe deformation device 39 produces compensation forces to return thesupport 20 to its initial state.

Nevertheless, it can happen that integration of the mirror 37 generatessmall errors of angle and small local deformations of the mirror, of theorder of a few microns, in the vicinity of its anchor points.

These errors can be compensated by performing new measurements byscanning the topography of the mirror 37. The difference between themeasured topography and the desired topography makes it possible todetermine the quantity of material to be removed by ion machining, andthus to adjust the parameters of the ion machining. The measurementsystem 30 is then moved away and the machining head is put into place.It has an X, Y, Z positioning device for positioning the machining headrelative to the reference cylinder 33. In a variant, the machining headcan be mounted on the device for making measurements by scanning, whichmakes it possible to perform such machining immediately after the stepof measuring topography.

It is also possible to provide a subsequent coating step, as mentionedabove, made up of one or more layers, in particular metallic layers ororganic layers. The coating head can be installed on the machining head,in which case the assembly can be part of a robot suitable forperforming all of the operations (measuring topography, machining,coating) without interrupting the vacuum, thereby achieving optimumcleanness, and achieving significant time savings.

It is also possible at all times to test a mirror or a set of mirrorsoptically on a vertical axis, thereby minimizing deformation due togravity, and to do so at various wavelengths using the proceduredescribed in the article by J. P. Collette et al. “Performance of XMMoptics and vertical test facility”, SPIE Proceedings, Denver, 1996.

Once a mirror has been integrated, it is possible to integrate thefollowing mirror by repeating the entire sequence.

The support 20 can be tilted by a tilt device 38 for systems, inparticular open-surface mirror systems, in which two successive mirrorscan present different angles between their optical axes and thevertical.

The scanner device 30 can be as shown in FIG. 3. It has a main plate 31fitted with a centering sensor 32 of the contactless type foridentifying the position of the plate 31 relative to the referencecylinder 33 standing on the support 20. The plate 31 is rotatable aboutan axis parallel to the axis 33′ of the reference cylinder 33, therebymoving the measurement head in azimuth. The azimuth angle is measured byan angle sensor. The main plate 31 carries at least one arm 34 that ismovable in translation along the longitudinal axis of the plate 31. Thearm 34 carries a measuring plate 35 which is mounted on a bench fittedwith two motors and which can be moved both vertically along thelongitudinal axis of the arm 34, and horizontally

The measuring plate 35 carries three sensors referenced A, B, and C. Thesensor A is a short-range sensor, e.g. of the laser type, the magnetictype, or indeed the capacitive type, and it faces the optical surface37′ of the individual mirror 37 that is being integrated. While theoptical surface 37′ is being scanned, the displacements of the plate 35are servo-controlled so that the distance d between the sensor A and thesurface 37′ remains constant, and thus so that the distance between themeasurement plate 35 and the surface 37′ remains constant.

The sensor B, e.g. of the laser type, serves to determine the distance Dbetween the plate 35 and the reference cylinder 33. The distance betweenthe optical surface 37′ of the mirror and the axis 33′ is thus equal tothe distance d plus the (constant) distance D₀ between the sensors A andB, plus the distance D, plus the radius r of the reference cylinder 33.

The sensor C, e.g. of the laser type, serves to measure the Verticaldistance between the measurement plate 35 and the support 20. Theazimuth angle, and the values delivered by the sensors B and C are readat regular intervals, thereby making it possible to determine the (x, y,z) coordinates of the corresponding point on the surface 37′ of themirror 37.

As mentioned above, a plurality of arms can be carried by the plate 35so that the assembly constitutes a robot for measuring, machining, andcoating.

The plate 35 can carry an arm including the machining head, the coatinghead, and sensors B′ and C′ analogous to the sensors B and C. The sensorA is superfluous in the case given that at this time the topography ofthe surface of the mirror is known and positioning of the arm requiresonly values for the azimuth angle (provided by the plate 35) and data asmeasured by the sensors B′ and C′.

The method of the invention can equally well be applied in part tosurfaces that are not optical.

What is claimed is:
 1. A method of assembling an optical assembly havingan entrance longitudinal portion having an entrance end and an exitlongitudinal portion having an exit end and comprising N coaxial shellsforming individual mirrors having an internal reflecting surface, eachof which extends between said entrance and said exit ends and presents afirst diameter at said exit end and a second diameter that is greaterthan the first at said opposite, entrance end, the method comprising: 1)putting the exit end of the second shell situated outermost in theoptical assembly into place on a support; 2) putting the exit end of thesecond shell which immediately adjacent thereto in the optical assemblyinto place on the support inside the first shell; and N) putting theexit end of the Nth shell which is situated innermost in the opticalassembly into place on the support.
 2. A method according to claim 1,wherein at least one of said operations of putting a shell into placecomprises: a) positioning one of said shells on the support; b)measuring the topography of the inside surface of said shell aspositioned on the support; c) repositioning said shell on the support asa function of the result of said topography measurement; and c′) fixingthe position of said shell on the support.
 3. A method according toclaim 2, wherein said topographical measurement is implemented bydifferential measurement by scanning both the inside surface of saidshell and a reference cylinder placed on the support in a referenceposition, said differential measurement being performed without makingcontact by means of sensors which are carried by a measurement platewhose displacements are identified relative to said reference cylinder.4. A method according to claim 2, wherein at least one of saidoperations of putting a shell into place comprises, after said fixing onthe support: d) measuring the topography of the inside surface of saidshell fixed on the support; and e) ion machining the inside surface ofsaid shell as a function of the result of said topography measurement.5. A method according to claim 4, comprising, after step e): f) applyinga reflective coating on the inside face of said shell.
 6. A methodaccording to claim 5, comprising, after step f): g) optically verifyingsaid shell.
 7. A method according to claim 1, wherein at least one shellpresents at least one extension integral with the shell and constitutinga mechanical fixing element, which is placed at least one of itslongitudinal ends.
 8. A method according to claim 7, wherein at leastone of said extensions disposed at one of said longitudinal endsconstitutes a baffle for attenuating parasitic light interference.
 9. Amethod according to claim 1, wherein at least one shell is constitutedby a plurality of elements extending between the entrance and exit endsof said shell, each element occupying a portion of the outline of saidshell, and wherein said elements present at least one extension integralwith the shell and constituting a mechanical fixing element which isdisposed at least one of the longitudinal ends thereof and at least oneof the side edges thereof.
 10. A method according to claim 8, wherein atleast one of said extensions disposed at one of said longitudinal endsconstitutes a baffle for attenuating parasitic light interference.